Herbicides are chemicals used to kill weeds (unwanted plants) that tend to compete with the grown crops. Herbicides can be selective or non-selective. Each of these groups can be sub-divided into three namely;
These are weed killers which by their chemical composition are capable of acting bio-chemically on either broad-leaved weeds or grasses. They are used predominately in monocropping situations.
However, when these herbicides are used in high concentration, they tend to have phytoxic effect on cultivated crops. Selective herbicides that kill grasses only are Alachlor, Pendimenthalin etc. also those that kill broad leaves weeds only are Atrazine, Imasaquine etc.
Examples of selective commercial herbicide are:
- Basamaize for maize.
- Ronster for rice.
- Galex for cowpea, soyabeans, groundnut.
- Basegram for rice and maize.
- Cotoran for cassava.
- Primextra for maize, cassava and yam.
- Diuron for yam and cassava.
These are otherwise known as total weed killers. They are used for weed control in any weed combination in the field, they will kill any kind of vegetation. Example is Glyphosate (Round-up).
CLASSIFICATION OF HERBICIDES BY MODE OF ACTION
- CONTACT HERBICIDES: These are types of herbicides kill any vegetation or weeds immediately after application. They stop all biological activities. E.g paraquat, gramoxone, basta etc.
- SYSTEMIC HERBICIDES: These are slow action type whose effects are not noticed until few days or week after application. Their effect is a thorough but gradual killing of most stubborn and perennial weeds. They have the ability to move around the transportation and translocation systems to achieve a total kill e.g Glyphosate.
- RESIDUAL HERBICIDES: These are soil acting weed killers whose effects on weed control are primarily as photosynthetic inhibitors. They can have both pre and post emergence activity; they are applied to the soil after cultivation and effectively control germinating weed seeds. Examples are zimazine, Diuron etc.
CLASSIFICATION OF HERBICIDES BASED ON TIME OF USAGE
- PRE-PLANTING HERBICIDES: This can be a foliar; selective or non-selective herbicide, applied before the crop is planted. Examples are Glyphosate , Paraquat.
- PRE-EMERGENCE HERBICIDES: These are applied in the soil before the emergence of weeds, immediately after cultivation e.g Galex + Metolachar used for cowpea, Cotoran used for cassava etc.
- POST EMERGENCE HERBICIDES: These are applied after the emergence of weeds and can have both contact and systemic actions, but usually selective so as not to have phytoxicity effect on cultivated crop. Examples are Laddox in maize, Paraquat, Propanil, Glyphosate.